After a well has been drilled, it must be completed before it can produce gas or oil. Once completed, a variety of events may occur to the formation causing the well and its equipment to require a “work-over.” For purposes of this application, “work-over” and “service” operations are used in their very broadest sense to refer to any and all activities performed on or for a well to repair or rehabilitate the well, and also includes activities to shut in or cap the well. Generally, work-over operations include such things as replacing worn or damaged parts (e.g., a pump, sucker rods, tubing, casings, and packer glands), applying secondary or tertiary recovery techniques, such as chemical or hot oil treatments, cementing the wellbore, and logging the wellbore, to name just a few. Service operations are usually performed by or involve a mobile work-over or well service rig (collectively hereinafter a “well service rig” “service rig” or “rig”) that is adapted to, among other things, pull the well tubing, rods or casings (referred to collectively hereinafter as “tubing”) and also to run the tubing back in to the well. Typically, these service rigs are mobile, such that they are capable of being moved from one location to another. The well service rig also typically includes extendible, jack-up derrick complete with draw works and block.
During tubing removal, a rig operator typically lifts a stand of tubing (or rods) which is then held in place by slips (or elevators for rods) while the stand is separated from the remaining portion of the tubing or rod string in the well. Once the stand of tubing has been separated from that which is still in the well, the stand of tubing can be placed on a tubing board, rack, or leaned up against the rig in some form or fashion. The process is substantially in the reverse for the insertion of tubing. During a lifting (pulling out of the hole or “POH”) or lowering (running in the hole or “RIH”) operation, the weight or load on the hook or block (typically referred to as a “hookload”) can fluctuate greatly based on the weight of the tubing string in the well, the conditions within the well, the condition of the tubing string, and the amount of acceleration of the tubing string. The hookload can increase very quickly to a level that is above the safe operating level of the rig. While alarms can be employed, if the operator cannot act quickly enough, the rig may be damaged and workers around the well could be injured.
In addition, as the stands of tubing are being pulled out of the well, the total hookload on the string is reduced. Conversely, as stands of tubing are added to the current tubing string and inserted into the well, the total hookload increases. In each case, while a single hookload limit might be effective during certain portions of the run, as the ratio of the hookload limit to the total hookload on the block changes, its effectiveness at protecting the rig operator, platform workers and the rig itself can be reduced.
Furthermore, as the hookload fluctuates during POH or RIH activities, so too does the overall weight of the rig (generally referred to herein as “measured weight” for the rig). The measured weight is typically a combination of the weight of the rig itself, the hookload and the weight of any tubing that is being stored on the rack or leaning against the rig. Typically drilling and well service rigs have weight limits under which they should be operated. As an RIH or POH activity is conducted, the measured weight for the rig can fluctuate and the limits for the measured weight for the rig during operation (referred to herein as “measured weight limit” or “MWL”) may need to be adjusted throughout the activity.